Surgical alteration of the shape of a patient's cornea to improve visual acuity has been practiced for a number of years. In such surgery, radial cuts are made into the surface of the cornea so as to alter its external curvature and, thus, its focal distance. Such surgery is termed radial keratotomy. More recently, excimer lasers have been employed to ablate sections of the patient's cornea to modify its optical properties. Recent advances in excimer laser keratectomy provide a surgical precision that challenges the ability to localize corneal pathologies with a high degree of accuracy. Before employing such laser-based surgical procedures, it is vital to know with precision, the thickness of individual corneal layers so as to assure the application of proper levels of laser power.
The prior art teaches a variety of techniques for imaging a cornea's structure. In U.S. Pat. No. 3,404,936 to Bennett et al, an ophthalmic instrument is described that enables measurement of the curvature of the cornea. The Bennett et al instrument is mounted on a curved support and enables a number of optical readings to be taken so as to chart the cornea's curvature.
Another method for determining corneal thickness and curvature is termed Pachymetry and involves the use of an ultrasonic probe to provide an A-scan image of various portions of a corneal surface. U.S. Pat. No. 4,817,432 to Wallace et al., U.S. Pat No. 4,546,773 to Kremer et al., U.S. Pat. No. 4,508,121 to Myers, U.S. Pat. No. 5,056,522 to Matsumura et al. and U.S. Pat. No. 5,029,587 to Baba et al., all describe various Pachymeter systems that provide A-scan corneal imaging and thickness measurements. Wallace et al employ a hand held probe, whereas the remaining patents show fixed ultrasonic transducers that provide ultrasonic corneal thickness measurements. U.S. Pat. No. 4,564,018 to Hutchison et al. and U.S. Pat. No. 4,930,512 to Henrikson et al. each disclose details of hand-held ultrasonic probes that enable ocular measurements to be obtained. The probes described by Hutchison et al. and Henrikson et al. require direct application of the probe structure to either a corneal surface or to an eyelid.
The prior art also includes teachings of the use of scanned ultrasonic transducers for the production of B-scan presentations of ocular structures. U.S. Pat. No. 4,484,569 to Driller et al. shows the provision of first and second transducers that are coaxially mounted. The first transducer enables both A and B-scan presentations to be obtained of an ocular structure, with the second transducer being employed for therapeutic use. U.S. Pat. 4,858,124 to Lizzi et al. also describes a fixed ultrasonic transducer whose beam is scanned and thereby creates a pie-shaped B-scan for ocular imaging (see FIG. 4).
U.S. Pat. No. 4,932,414 to Coleman et al. shows a B-scan ultrasonic transducer that is angularly mounted about a fixed axis therapeutic transducer. The B-scan transducer is rotatable about the therapeutic transducer and creates a pie-shaped sector scan and is movable so as to produce multiple B-scan presentations.
Ultrasonic B-scan presentations have also been employed to image intraocular tumor volumes by three-dimensionally scanning an ultrasonic transducer head. In one such system a conventional sector scanner is rotated around its axis. (See Hansen et al "Ultrasonographic, Three-dimensional Scanning for Determination of Intraocular Tumor Volume", Acta Ophthalmologica, 1991, 69, pages 178-186. Sherat et al have also disclosed the use of a B-scan ultrasound microscope for producing B-scan images of ocular structures (See Ultrasonic Imaging, Volume 11, pages 95-105, 1989). In the Sherat et al structure, a transducer is mounted in a liquid-filled tank and is moved about the surface of the tank so as to obtain an image of a specimen that is positioned within the tank.
Coleman et al. in "Ultrasonography of the Eye and Orbit", Lea and Febiger, 1977 pages 51-88, describe a number of ultrasonic systems that provide both A, B, and M mode systems. At pages 65-69, various scan pattern are considered, i.e. linear, sector, arc, and compound combinations thereof. Coleman et al. also indicate that the most useful patterns are those in which the ultrasonic beam is perpendicularly aligned with reflective tissue surfaces, in that the echoes travel directly back to the transducer rather than being redirected along misaligned axes. Coleman et al. do not specifically teach how to obtain such perpendicular alignment over a complete corneal surface.
As is evident from the above, ultrasound has been widely employed to image both ocular and corneal structures. However, with the onset of excimer laser kerotectomy, extraordinarily precise measurements of corneal thickness over a cornea's entire surface are required. At a minimum, a highly accurate B-scan image of the corneal structure must be obtained. Furthermore, it is important to be able to specifically localize areas of opacity in the cornea. In order to achieve optimum imaging of corneal structures, the reflected signals must be as noise free as possible and spurious reflections must be minimized. Due to the highly reflective properties of the corneal surface, such high quality signals are difficult to achieve.
Accordingly, it is an object of this invention to provide an improved ultrasound scanner for obtaining ocular images.
It is another object of this invention to provide an improved ultrasound scanner that is able to achieve highly accurate corneal thickness measurements.
It is still another object of this invention to provide an improved ultrasound ocular scanner which provides enhanced ocular images without requiring direct transducer contact with the cornea.